Jacques and George meet in the middle of a lake while paddling in their canoes.They come to a complete stop and talk for a while.When they are ready to leave,Jacques pushes George's canoe with a force to separate the two canoes.What is correct to say about the final momentum and kinetic energy of the system if we can neglect any resistance due to the water?

Question

Jacques and George meet in the middle of a lake while paddling in their canoes.They come to a complete stop and talk for a while.When they are ready to leave,Jacques pushes George's canoe with a force  to separate the two canoes.What is correct to say about the final momentum and kinetic energy of the system if we can neglect any resistance due to the water?

Quizplus · Accepted Answer

By Newton's Third Law, the force applied by Jacques on George's canoe is equal in magnitude and opposite in direction to the force applied by George on Jacques' canoe. Therefore, the net external force on the system (the two canoes) is zero, and the total momentum of the system remains constant. Since the initial momentum of the system was zero (the two canoes were at rest), the final momentum of the system must also be zero. However, since Jacques applied a force on George's canoe to separate them, there was work done on the system, and thus the total kinetic energy of the system was not conserved. Therefore, the final kinetic energy of the system is zero, which means that the canoes come to a complete stop. Thus, the correct choice is B, which states that the final momentum is in the direction opposite of the force applied, and the final kinetic energy is zero.

Jacques and George Meet in the Middle of a Lake